Khan was not a recast. Why? Because RM was such a force in the original role, that no one else would be able to replicate his aura and unique delivery. Bennet, et al were correct, as opposed to pulling a Dick Sargent--which no one appreciated.
Actually, if I recall correctly, "Space Seed" was selected after Bennett screened all 78 one-hour episodes of TOS looking for the best one with sequel potential - and quickly latched onto that one. They had no idea
if Montalban was interested - or even available - since he'd become synonymous with the white-suited Mr Roarke ("Fantasy Island") - Madlyn Rhue even guested on that show
- and they didn't write the Khan synopsis on the assumption that Montalban would be reprising the role. Casting the strongest actor in their (meagre) price range was their goal, and they were pleasantly surprised when Montalban was amenable.
Recasting was not necessarily a big deal. Remember that one plan for ST:TMP, that very nearly could have happened, was Paul Newman and Robert Redford as Kirk and Spock (or Spock and Kirk?), and Walter Koenig guesting as Chekov's father.